FIG. 1 is a systematic block diagram of a lamp ballast for use with gas discharge lamps according to the prior art. As shown in FIG. 1, the lamp ballast for use with gas discharge lamps includes a power factor correction converter (PFC converter) 102 and an inverter 104 for igniting and operating gas discharge lamps LP1-LP2. The power factor correction converter 102 is typically an active boost converter and the inverter 104 is typically a self-oscillating parallel resonant circuit. The capacitors Cb1 and Cb2 are connected in series with gas discharge lamps LP1-LP2, respectively, for balancing the lamp currents flowing through the gas discharge lamps LP1-LP2. The conventional lamp ballast can be classified into two categories according to the ignition method of the gas discharge lamp. The first category is called pre-heating lamp ballast, and the second category is called instant start lamp ballast. For a pre-heating lamp ballast, a considerably high voltage is needed to be applied to both sides of the gas discharge lamp to pre-heat the filament of the lamp before the gas discharge lamp is ignited. The heating power for the filaments of the gas discharge lamp is generally provided by the inverter 104. The pre-heating mechanism for the filaments of the gas discharge lamps is achieved by coupling a number of heating windings (not shown) to the transformer of the inverter 104 (not shown). After the inverter 104 is started, the heating windings will generate heat by way of electromagnetic effect in order to pre-heat the filaments of the gas discharge lamps LP1-LP2. Nevertheless, before the filaments of the gas discharge lamps LP1-LP2 are fully heated, an output voltage is established across the gas discharge lamps LP1-LP2. This output voltage would result in a glow discharge current. Another drawback of the conventional pre-heating mechanism for gas discharge lamp is that the heating power is hard to be removed after the gas discharge lamp is operating in a stable state. This would increase the power loss of the lamp ballast.
Therefore, it is preferable to provide a filament heating apparatus for pre-heating the filaments of gas discharge lamps in order to improve the efficiency of gas discharge lamps.